Seismic sway bracing and branch line restraints in SDC A & B per NFPA 13 (2013)

Seismic sway bracing and branch line restraints in SDC A & B per NFPA 13 (2013)

I wanted to get back to this old thread now that it is 2016, 3 yrs after NFPA 13 (2013 print) has been out.

I'm a licensed structural engineer (SE) working with a fire protection engineer (PE) in a project located in TX (that's actually irrelevant) in Seismic Design Category (SDC) "B". What is the current interpretation of the requirements of seismic sway bracing and branch line restraints for sprinkler systems? Please consider the following points:

1.) ASCE 7-10 designates sprinklers as life-safety purpose after earthquake in Sect. 13.1.3 and therefore it does NOT fall under Exemptions per ASCE 7-10 13.1.4, "Item 4. Mechanical & Electrical components in Seismic Design Category B" is not applicable for sprinklers since Section 13.6.8.2 of ASCE 7-10 references NFPA 13 as deemed to meet the requirements of force & displacements of ASCE 7. So far so good!

2.) NFPA 13 (2013) does not make any distinction whatsoever about seismic design categories (SDC). NFPA 13 Section 9.3.1.1 states "Where water-based fire protection systems are required to be protected against damage from earthquakes,the requirements of Section 9.3 shall apply. Who sets this requirement? IBC 2012 does not, it creates a circular reference back to ASCE 7 which goes to NFPA 13.

3.) NFPA 13 (2013) is a Standard, not a legal document (Code). International Building Code (IBC), i.e., the 'Code' with pertaining legal requirements, in Section 903.3.1.1 states: "Where the provisions of this code require that a building or portion thereof be equipped throughout with an automatic sprinkler system in accordance with this section, sprinklers shall be installed throughout in accordance with NFPA 13." There is no other mention of seismic bracing for sprinklers in IBC 2012. Further, 1613.1 Scope "Every structure, and portion thereof, including nonstructural components that are permanently attached to structures and their supports and attachments, shall be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7". Again, no specific requirements for sway braces and/or branch line restraints for any SDC.

4.) NFPA 13 (2013) Section 9.3.5.9 provides a conservative yet simplified approach to calculate the horizontal seismic loads for the braces. It is using a Cp factor which in turn is tied into the short period response parameter Ss. It lists values of Cp for areas with Ss less than 0.33 with Cp = 0.35 (i.e., 35% of water filled pipe weight is the horizontal seismic force). This would imply that any area of the US, irrespective of the SDC (A or B and higher) would require seismic bracing for sprinklers since there are no areas where Ss=0 on the USGS maps. In other words, even SDC A and B would need to be designed for a minimum of Fp = Cp*Ww where Cp = 0.35.

In ASCE 7-10, sprinklers fall under "mechanical and electrical components" and §13.1.4 exempts all mechanical and electrical components in Seismic Design Category (SDC) B from the seismic requirements (including life safety equipment). Since fire sprinklers are life-safety equipment, they are assigned and importance factor, Ip = 1.5. Note that §13.1.4 exempts mechanical and electrical components in SDC C with Ip=1.0 from the seismic requirements. Unless NFPA 13 or the local building code says something different, sprinklers in SDC A and B are exempt.

If there is anywhere I am really weak it's when it comes to seismic design because in over 40 years only had to do it once and that was back in the late 1980's.

If I had to guess more than 95% of the work I have done Ohio, Michigan, western Pennsylvaniar, a little bit in Kentucky just south of Cincinnati and a couple large jobs I did in Bowling Green, Kentucky. As far as working south nearly everything I have done has been south of Macon, Georgia between the coast and midway across Alabama and a tad in northern Florida.

I guess I could say the areas I've worked in aren't exactly those areas that rock and roll.

Question: Where can I find a good map that shows areas of seismic design categories? A bit ago I bid a job in Auburn, Alabama and and looking at it it might require seismic but I am not sure.

What do I do if I discover the job in Auburn needs seismic? Give my deer in the headlights look I guess.

Another thing I would like to know is what about a rigid metal frame and bent metal purlin building the sort I deal with a lot? I guess in areas around the rigid frames it is easy enough even I could do it but what about end walls that do not have expansion frames? All you got is light gauge metal, what do you do here? Frustrating because everything I've seen shows it with concrete structures and that makes it easy.

But to start out can anyone point me to a detailed map that will show me where zones A, B, C and so on actually are?

the interpretation is somewhat vague in ASCE 7-10 exemptions. It surely does not say all mech & elect components.

However more importantly, ASCE is a engineering guideline, not a legal document or "Code" like IBC is. IBC 2012 requires NFPA 13 compliance for sprinklers, and NFPA 13 does not have exemptions -- see my point 3) in the original thread.

I do not want to require seismic bracing or BLR's in SDC B (or A) but I cannot justify how I am not required to do so. White papers by FM suggest to the owners that sprinkler system are braced to aid in post-earthquake fires (however small) and to lessen the potential of flooding the rooms with water with burst pipes.

I believe there are other threads on this. There are no Seismic Category maps, only USGS ground motion parameters for 0.2 sec and 1.0 sec spectral response acceleration values in Ch22 of ASCE 7-10. With this information AND additional parameters (risk occupancy/risk category and soil conditions), one will perform a series of steps outlined in ASCE 7 to arrive at Seismic Design Category (SDC).

Quote:

AdamU (Structural)
(OP)
8 Aug 16 14:10
wannabeSE,

the interpretation is somewhat vague in ASCE 7-10 exemptions. It surely does not say all mech & elect components.

However more importantly, ASCE is a engineering guideline, not a legal document or "Code" like IBC is. IBC 2012 requires NFPA 13 compliance for sprinklers, and NFPA 13 does not have exemptions -- see my point 3) in the original thread.

That's not true. Look at 2012 IBC Chapter 35 and ASCE 7-10 is referenced in about 30 sections of the building code. So it has the force of law as does NFPA 13 when it adopted by a governmental jurisdiction. I will say that when you look at the specific IBC section, in certain instances the provision only references specific ASCE 7 sections or Chapters. But at the end of the day, the Building Official is the approving authority under the IBC and he/she can enforce certain sections of ASCE 7 - 2010 edition.

I agree, ASCE 7 is referenced by IBC and it becomes "legal" by this means. However, ASCE 7 alone does not have legal merit. But I don't want to diverge from the original thread. My question remains:

If IBC Section 903.3.1.1 states: "Where the provisions of this code require that a building or portion thereof be equipped throughout with an automatic sprinkler system in accordance with this section, sprinklers shall be installed throughout in accordance with NFPA 13. NFPA 13 does not have exemptions for Seismic Categories A or B, in fact it does not mention them at all. NFPA 13 requires sway braces & BLR's across the board with force in it varying per item 4) in the original post. How can I justify the aforementioned exemptions for SDC A & B?

I am not a structural engineer but I think your answer is in 2012 IBC Section 1613.1 and ASCE 7. Essentially one must look at the mapped acceleration parameters, the site's soil class, and make any of the IBC required adjustments. The final answer is in IBC Section 1613.3.5.

The 2009 IBC had a statement that everyone is referencing which essentially said "if your building is in Category A or B SDCs, have a nice day when evaluating sprinkler systems because you don't need to worry about." That provision has gone away. So essentially, 2012 IBC Section 1613.1 requires the Structural Engineer to make the determination.

I think you're starting to see where I am coming from as a structural engineer. I too am inclined to say SDC A & B does not need seismic bracing. But so far in this discussion I don't see a clear statement of interpretation in IBC (and by reference to ASCE 7 and NFPA 13) that starting with IBC 2012 I can confidently state sprinklers are exempt from bracing on mainlines and restraints on branches. And unfortunately, I need to prove it to the authority having jurisdiction (AHJ) where the exemption is located. The fire protection drawings refer to NFPA 13, as they should since no FP engineer/MEP vendor is going to use ASCE 7. There, SDC's are not mentioned at all nor there are any exemptions for any SDC. Following NFPA, one will always arrive at horizontal forces to be supported by the structure [see item 4) of the original post]

The projects structural drawings should list the seismic design category. It is typically on the structural general notes sheet. If not, you can use a the USGS internet tools at http://earthquake.usgs.gov/designmaps/us/applicati.... You can search on an address. If that doesn't work, use google maps or similar to find the latitude and longitude (with google maps, right click on the map and select "what's here").For the site class in the tool use D if you don't know otherwise. The risk category is typically 1, 2 or 3; Essential facilities are 4 (hospitals etc). The tool generates a report with seismic accelerations Sds and Sd1. In the report click on "view the detailed report". Toward the bottom of the report it tells you the seismic design category. See attached

The only interpretation you will ever receive is from the code official. All we are doing in this medium is offering educated opinions. As I said earlier, the final decision will rest with the Building Official. In turn, the Fire Official will make a determination on compliance with NFPA 13 Chapter 9 based on the analysis required in 2012 IBC Section 1613. I suspect the provision that was in the 2009 IBC was deleted because someone made an assumption and some factor (Occupancy Risk Categorization or Soils) combined with USGS data on mapped acceleration parameters pushed a former SDC Category B design into a place where more robust bracing was required. Again, I'm speculating but the previous provision is no longer in the 2012 IBC.

And I live in South Texas so I don't ever worry about this. Even more fortunate is that the nearest hydraulic fracturing operations to my jurisdiction are 100 miles SW. Plus, the price of oil/barrel has put a dampener on drilling activities.

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